Method and apparatus for coating with plastics

ABSTRACT

Molten, thermoplastic resin is extruded from an annular die having a central hole through which an elongated article, such as a pipe is moved axially, so as to cost the article with hot resin. The hot resin coating is then precooled by a uniform stream of air issuing from an annular slot in a ring surrounding the coated article. The slot is inclined in the direction of axial movement of the article. The article then enters a similar ring also having a second inclined slot for discharging a uniform stream of cooling water onto the coated article. The direction and force of the stream of air deflects the sprayed stream of water so that it contacts the coated article in a substantial axial direction so that the contact line of the water spray is substantially tangential to the surface of the elongated article. Thereafter, the article enters through a gasketed opening in its side wall, of a tank containing a liquid to effect cooling of the coated article by immersion. The coating is free of defects due to prior deflection of cooling water preventing direct impingement and providing substantially tangential contact zones of the water on the article.

United States Patent Okamoto et a1.

METHOD AND APPARATUS FOR COATING WITH PLASTICS inventors: Akio Okamoto;Taiko Uno. both of s ids an.

Assignee: Mitsubishi Petrochemical Co., Ltd.,

Tokyo,.lapan Filed: Aug. 17, 1970 Appl. No.: 64,254

Foreign Application Priority Data Aug. 18, 1969 Japan ..44/64286References Cited UNITED STATES PATENTS Allan et al. ..264/95 Postlewaite..264/ 174 Jansson et a1 ..264/95 .Tornberg ..18/14 Houston ..1 18/69 1Sept. 26, 1972 Attorney-Martin Smolowitz [57] ABSTRACT Molten,thermoplastic resin is extruded from an annular die having a centralhole through which an elongated article, such as a pipe is movedaxially, so as to cost the article with hotresin. The hot resin coatingis then precooled by a uniform stream of air issuing from an annularslot in a ring surrounding the coated article. The slot is inclined inthe direction of axial movement of the article. The article then entersa similar ring also having a second inclined slot for discharging auniform stream of cooling water onto the coated article. The directionand force of the stream of air deflects the sprayed stream of water sothat it contacts the coated article in a substantial axial direction sothat the contact line of the water spray is substantially tangential tothe surface of the elongated article. Thereafter, the article entersthrough a gasketed opening in its side wall, of a tank containing aliquid to effect cooling of the coated article by immersion. The

coating is free of defects due to prior deflection of cooling waterpreventing direct impingement and providing substantially tangentialcontact zones of the water on the article.

FATENTEDSEPZB m2 3.694538 IN VE' N TORS AK/O OKAMOTO TA/KO UN 0 METHODAND APPARATUS FOR COATING WITH PLASTICS This invention relates tocoating articles with plastic resins, and more particularly to thecontinuous cooling of thermoplastic resin a hyphen coated elongatedarticles.

Heretofore, the cooling of thermoplastic resin extrusion mouldings hasbeen carried out by using air, water (direct and indirect) and thecombination thereof. With regard to a cooling method for thin-walledmoulding such as a tube or a sheet, various excellent methods have beendeveloped and the present requirements have been almost but not quitesatisfied.

However, cooling methods for extruding tubes, bars and other differentshapes having a thick-wall and an extrusion coating of plastic resinhave until the present invention been imperfectand low in efficiency.

In the past there has been an indirect method of cooling in which acooling ring, having a cooling plate of the same cross sectional shapeas the coated articles with cooling water circulated at the back surfaceof cooling plate, is allowed to be in contact with the extrusions andmoved while maintaining the contact relation to effeet an indirectcooling, and thereafter the extrusions are passed through a coolingwater bath. Although such .cooling method is suitable in the field ofproducts requiring exact size, its cooling ability is low and themoulding velocity is slow.

On the other hand, in a direct cooling method it is usual that eitherthe cooling water be poured on the extrudate as it emerges fromthe die,or alternatively the extrudate be passed through a cooling water bath,but it is difficult to prevent the leakage of water from the bath at theintroducing inlet of the extrudate and, if a sealant such as an elasticbody is used, the extrudate suffers from undesirable scratches becausethe extrudate is still soft. And, if the leakage of the cooling water issplashed on the surface of extrudate, which usually is a highlycrystalline fast crystallization velocity resin,

the resin is partially cooled and, as a result, partial shrinkage occursand uneven cooling patches clearly develop on the surface of theproduct, which greatly reduces the value thereof. If the distance fromthe die to the cooling water treatment location is extended, andextrudate is first subjected to gradual air cooling to a sufficientextent, and then is immersed in a cooling water, the aforesaiddisadvantages may be overcome, but other defects such as low efficiency,drawdown of resin and undesirable growth of crystal, result.

In extrusion thermoplastic resin products, which are produced by meltingand mixing thermoplastic resin materials in an extruder, and extrudingthe molten resin continuously through the nozzle thereof, the productsare still low in viscosity, so that they are apt to suffer from changesin shape and surface condition under the influence of the slightestchange in the open atmosphere. Therefore, it is necessary that themolten film as it is extruded from the nozzle should be cooled in amoderate and uniform environment without damaging the shape and surfaceof the products.

It is an object of the present invention to precool the coated articleswithout destroying the shape of the product under the condition where noblast pressure is exerted on the outer surface of a molten coating bypassing a current of cooling air along the surface of coated articlewhereby the shape thereof is prevented from being deformed by the actionof the dynamic pressure of the air, and to carry out sizing at increasedviscosity of the outer surface of the coated article under stableconditions.

It is a further object of the invention to provide a water cooling ringfor supplying cooling water uniformly onto the extrusion moulding fromthe circumferential direction into the axial direction and downstream ofthe cooling air blowing means (i.e. along the direction of thewithdrawal of the coated article) to further cool the precooled coatedarticle.

However, should the cooling water flowing to the coated articles throughthe water cooling ring directly impinge upon the coated film, thereoccur disadvantages particularly if the contact line of the coolingwater with the coated article is not tangential, with respect to thewithdrawal direction so that the cooling line falls into disorder, andwater drops impinge horizontally and vertically to adhere to the frontportion of the cooling water contact line of the coated article thuscausing cooling patches of a pitted shape develop.

It is, therefor, a further object of the invention to utilize thecooling air for the purpose of preventing the direct impingement of thecooling water upon the extrudate to present said faults.

That is, in accordance with the present invention the direct impingementof water drops can be prevented by blowing the cooling air under highdynamic pressure from the air blowing ring axially along the coatedarticle behind the stream of the cooling water so that the contact lineof the cooling water can be made substantially parallel to the axialfeed direction of the coated articles.

The invention provides a novel method for producing coated articleshaving a smooth surface with high efiiciency. Such method comprisesprecooling the coated articles by blowing air along articles which areobtained by coating the surface of long articles to be coated withmolten resins. The articles are then cooled by spraying a cooling waterthereon in such a manner that the contact line on the coated article ofthe cooling water is uniform and straight. The article is finallyimmersed in a water bath to complete the cooling and solidification.

The invention includes novel apparatus for coating elongated articlewith thermoplastics including thermoplastic resins such as low densitypolyethylene, high density polyethylene, polypropylene,polyvinylchloride. These materials are continuously coated on elongatedarticles such as tubes, bars and the like.

In the drawing,

The single FIGURE is a fragmentary view, partly in section and partly inside elevation, of apparatus illustrative of the invention in extrusioncoated pipe by way of example. Referring to the drawing, thermoplasticmolten resin 2, molten and milled in an extruder (not shown), iscontinuously extruded into the form of a tube in the atmosphere throughan annular nozzle 4 of an annular die 3 on column 2 at a velocity of 7to 20 m/min, while a steel pipe 5 moves through the center bore or holeH of the die 3 in the same direction as that of the extrusion of themolten resin 2 and thus is coated by said molten resin 2 near the headof the die 3. In this case it is preferred that the pressure in annularspace 6 enclosed by molten resin 2 the steel pipe and the die 3 bereduced in order to ensure that the operation of coating the steel pipe5 with the molten resin is smoothly and compactly effected.

The extrusion coated composite 7 thus produced is passed through a ring8 blowing cooling air 10 disposed in contact with or at a distance up toabout 100 cm from the die 3 and precooled by the smooth stream coolingair 10 blown from an annular slot nozzle 9 of the blowing ring 8. In thecooling air blowing ring 8 the cooling air is supplied in thecircumferential direction from a blower 11 through a manifold or buffertank 12 and pipes 13 for equalizing air flow volume and pressure.

The nozzle or slot 9 from which the cooling air is blown is conical inshape and is directed or inclined toward the feed direction of thecomposition 7 at an included angle of 0 to 30 with respect to the axisof movement of the composite 7. The cooling air 10 blown from the nozzle9 has a dynamic pressure of 100 to 600 mm H O (water column) a velocityof 15 to 50 m/sec, and the cooling air 10 travels along the composite 7parallel to the feed direction. Therefore, no negative pressure due tothe cooling air 10 occurs on the surface of the extrusion mouldingcomposite 7 which would result in the deformation of the shape thereof.

The extrusion coated composite 7, thus precooled by air, is passedthrough a water cooling ring 15 fixed in contact with or at a distancewithin 100 cm from the cooling air blowing ring 8 to further cool, bycooling water 17, flowing from an annular slot or nozzle 16 of the watercooling ring 15. In order to spray the required amount of the coolingwater, at the predetermined pressure into the water cooling ring 15, thecooling water is supplied through pump 18, and a pipe 19. The nozzle 16from which the cooling water is discharged, is conical in shape and isalso directed along the axial feed direction of the composite 7 at anangle of to 60.

Therefore, the cooling water 16 from the water cooling ring 15 is blownby the cooling air 10 from ring 8 toward the surface of the composite 7at an angle of 30 to 60, and on the other hand, since said cooling air10 is blown on the stream of the cooling water 16, the cooling water 16is prevented from irregular impingement upon composite 7.

As a result the surface of composite 7 has a uniform appearance.

In order to further increase the cooling effect, the extrusion coatedcomposite 7 moves into a liquid cooling bath 21 passing through a sidewall opening having an annular packing 20 in tank T. In such immersionthe article is completely cooled. A water bath is preferred Also, withproducts which require exactness in product size, it is possible to usethe present invention in combination with such instruments such assizing dies. But, in such case the length between the air and waterrings 8, 15 may be shorter than that hitherto used.

When other type mouldings are to be coated of cross section that is arectangle, ellipse, or rhomb, it is not necessary that the shape of theair cooling ring 8 and the water cooling ring 15 exactly conform to suchshapes. The opening may be allowed to align with that of the mouldingsto be coated, and the circular opening having a diameter in the range ofthe dimension substantially, equal to the largest dimension to up to 100mm may be used as substitutes. In the preferred embodiment the distancebetween the die and the cooling air blowing ring is 0 to 30 cm, and thedistance between the cooling air blowing ring and the water cooling ringis 10 to 20 cm and further the distance between the water cooling ringand the water bath is 30 to 50 cm.

EXAMPLE With respect to the coating of 8.6.1. steel pipe 3 inches in theoutside diameter with polypropylene (Melt Index 10), the followingexperimental results were obtained as shown in Table 1. In the case ofA, which corresponds to the method of the present invention, coating iseffected by using the apparatus shown in the drawing wherein thedistance between the die and the cooling air blowing ring, the distancebetween the cooling air blowing ring and the water cooling ring and thedistance between the water cooling ring and the bath are respectivelydetermined at 25 cm, 15 cm and 40 cm. In the case of B, coatingiseffected by using the apparatus shown in the drawing and the distancebetween the air blowing ring and the water bath is 1.5m. And in the caseof C, coating is effected by using the apparatus wherein the watercooling ring is removed from the apparatus shown in the drawing and Theterm film shrinkage rate is intended to signify the shrinkage amount ofthe film of the coated steel pipe after coating and cooling have beencarried out and the coated steel pipe has been cut into a length of 5m.Small is 10 mm in shrinkage amounts and practically preferable, medianis 10 to 20 mm in shrinkage amount and practicable, and large is 20 mmshrinkage amount and impracticable.

The term product appearance is intended to signify the uniformity of thecoated film observed from appearance, and good represents that the filmas a whole is smooth, and bad" represents that the unevenness owing tothe unequal shrinkage of the coated resin is present.

In addition, the conditions under which said experiments have beencarried out are as follows;

(1) Materials used Polypropyelen (ML= 1.0)

Steel pipe the outside diameter 3 inches (2) Instruments used Extrudermm qb Die caliber mm d) (3) Operation conditions Resin extrudingtemperature 240C Cooling I 8C C The invention not only eliminates theproblems and troubles of the prior art, but provides an excellent methodfor cooling thermoplastic coated articles, leaving them unspoiled and inperfect condition.

What is claimed is:

1. Apparatus for coating elongated articles with thermoplastic resinswhich comprises means for extruding molten plastics onto the surface ofsaid articles to be coated, to form a coating thereon,

means for moving said coated articles in the direction of their majoraxis,

' air blowing means for blowing cooling air along and substantiallyparallel to the surface of the moving coated articles,

a water cooling spray means for spraying cooling water on the coatedarticles closely adjacent said air blowing means so that water passingfrom said spray means is deflected by the air from said blowing means ina direction substantially parallel to said moving articles, and

a water cooling bath for immersing the coated articles.

2. Apparatus according to claim 1 wherein,

said cooling air blowing means is a ring with an annular nozzle directedalong the axis of movement of the coated articles at an angle of 0 tothereto and is conical in shape.

3. Apparatus according to claim 1 wherein,

said water cooling means is a ring that has an annular nozzle directedin the axis of movement of the coated articles at an angle of 30 to 60thereto, and is also conical in shape.

4. The method for coating elongated articles with thermoplastic resinswhich comprises coating the outer surface of said articles with moltenthermoplastic resin and solidifying the resin coat by cooling withcooling air from a cooling air blowing ring blown along the surface ofthe coated articles to pre-cool the same, and then with cooling waterfrom a water cooling ring said air being blown along the surface of thearticle toward the cooling water so that the cooling water is preventedfrom directly impinging on the coated articles, and the contact line ofthe cooling water on the coated articles is thereby maintainedsubstantially tangent with respect to the surface of the elongatedcoated articles along its major axis and subsequently introducing saidarticles into a bath of cooling water.

5. Method of coating an elongated article with thermoplastic resinaccording to claim 4, which comprises, extruding molten resin through anannular die surrounding the article to form a coating thereon, movingthe coated article axially in the direction of its length through a holein the axial center of the die, while such resin is so extruded, to coatthe article, blowing cool air, from an annular slot in a first ringsurrounding such coating, in a direction substantially parallel to thedirection of movement onto the moving surface of the resulting coatingon the moving article, said slot being inclined in the direction ofmovement of said coated article and then spraying cooling water from asecond annular slot in a second ring surrounding said coated articlepositioned downstream with respect to said first ring, such second slotbeing also mchned 1n the direction of movement of the coating wherebythe air blown from said first ring deflects the water from said secondring in a direction tangential to said coating moving surface to preventdirect impingement of said water upon said coated surface andsubsequently introducing said article into a bath of cooling water.

6. The invention as defined by claim 5, in which the coated article issubsequently introduced into a bath of cooling water by moving samethrough a gasket-sealed opening in the side wall of a cooling watertank.

2. Apparatus according to claim 1 wherein, said cooling air blowingmeans is a ring with an annular nozzle directed along the axis ofmovement of the coated articles at an angle of 0 to 30* thereto and isconical in shape.
 3. Apparatus according to claim 1 wherein, said watercooling means is a ring that has an annular nozzle directed in the axisof movement of the coated articles at an angle of 30* to 60* thereto,and is also conical in shape.
 4. The method for coating elongatedarticles with thermoplastic resins which comprises coating the outersurface of said articles with molten thermoplastic resin and solidifyingthe resin coat by cooling with cooling air from a cooling air blowingring blown along the surface of the coated articles to pre-cool thesame, and then with cooling water from a water cooling ring said airbeing blown along the surface of the article toward the cooling water sothat the cooling water is prevented from directly impinging on thecoated articles, and the contact line of the cooling water on the coatedarticles is thereby maintained substantially tangent with respect to thesurface of the elongated coated articles along its major axis andsubsequently introducing said articles into a bath of cooling water. 5.Method of coating an elongated article with thermoplastic resinaccording to claim 4, which comprises, extruding molten resin through anannular die surrounding the article to form a coating thereon, movingthe coated article axially in the direction of its length through a holein the axial center of the die, while such resin is so extruded, to coatthe article, blowing cool air, from an annular slot in a first ringsurrounding such coating, in a direction substantially parallel to thedirection of movement onto the moving surface of the resulting coatingon the moving article, said slot being inclined in the direction ofmovement of said coated article and then spraying cooling water from asecond annular slot in a second ring surrounding said coated articlepositioned downstream with respect to said first ring, such second slotbeing also inclined in the direction of movement of the coating wherebythe air blown from said first ring deflects the water from said secondring in a direction tangential to said coating moving surface to preventdirect impingement of said water upon said coated surface andsubsequently introducing said article into a bath of cooling water. 6.The invention as defined by claim 5, in which the coated article issubsequently introduced into a bath of cooling water by moving samethrough a gasket-sealed opening in the side wall of a cooling watertank.